Treatment of rustless iron



Patented Jan. 4, 1938 UNITED STATES PATENT OFFICE mesne assignments, to Rustless Iron and Steel Corporation, Baltimore, Md., a corporation of Delaware No Drawing. Application April 15, 1933, Serial No. 666,390

5 Claims. (01. 148-65 This invention relates to rustless iron and steel products and more particularly to an art of treating the surfaceof the same.

Among the objects of my invention are the production of a rustless or corrosion resistant iron or steel article, product or manufacture which is treated in a simple, inexpensive, thoroughly practical and highly emcient manner whereby the surfaces of the article, product or .manufacture are rendered particularly free from the inception and growth of rust orcorrosion thereon under the varying conditions of actual practical use.

The invention accordingly consists in the several steps and in the relation of each of the same to one or more of the others as described herein and the scope of the application of which is indicated in the following claims.

As conducive to a clearer understanding of certain features of my invention it may at thispoint be noted that in heretofore known and/or used corrosion-resistant iron or steel products such as valves, pipe-lines, evaporators, crystalizers and the like as well as products or articles of a more decorative nature such as ornamental iron-work, decorative metal trim, fancy hardware and novelties where minute rough, sharp. or jagged surfaces are presented as a result of cutting, stamping, forging, deep-drawing, or other forming of the metal stock to give a product of the desired size and shape, rust or corrosion forms and progresses over the surface -of the product under the conditions of actual use, the rustless character of the product notwithstanding.

The corrosion usually initiates at the various minute rough, sharp or jagged surfaces presented and gradually progresses until a large portion of the surface of the product is covered, thus deco ilar oxide coating. This attach, however, is

tracting from the appearance of the product as well as from-the utility of the same.

In heretofore known and/or practiced methods of preventing the corrosion of corrosion-resistant products of the character described, the

product is dipped in a bath of an oxidizing agent,

such as a strong solution of nitric acid, to clean and passivate the surface as a final step in finishing the product foruse. The oxidizing agent directly attacks the surface of the product forming thereon an oxide coating.

It is found, however, that under the conditions of actual use such a protective coating is soon penetrated, thus permitting corrosion of the metal, all as more particularly indicated above.

The use of other passivating solutions such as a strong solution of sulfuric acid and chromic acid vigorously attack the surface of the corrosion-resistant product and form thereon a simaccompanied by a minute pitting or etching of the surface which detracts from the utility of the product, when slight changes in size are of importance, and alters the surface quality of the product as well (a polished surface is destroyed). In addition this action is accompanied by a depletion of the active agents in the solution, an increase in the quantity of iron salts present and a resultant weakening of the solution which I impairs and ultimately destroys the effectiveness of the same.

One of the objects of my invention is to provide "an art or process for treating corrosion-' product without deleterious attack on the surface of the product and without depleting or exhausting the solution, and which renders the surfaces ofsuch products non-corrosive over long periods of constant use.

Referring now more particularly to the manner of practicing my invention, a corrosion-resistant iron or steel product, illustratively rustless iron automobile trim, is first'manufactured as by cutting, deep-drawing and trimming rustless iron sheet stock of a desired analysis such as, for example, 18% chromium, 8% nickel, .l5%

carbon, with small amounts of manganese and silicon, and traces of sulfur and phosphorus.

and/or polishing when a desired surface quality is obtained. I v

To prevent corrosion of the surface of the article or manufacture (the formation and progressive growth of the corrosion product), the rustless iron manufactureis nowgiven a. passivating treatment. j

The product, article "or manufacture, conveniently as a last step just prior to wrapping or packaging for shipment, is plunged into a weak, hot solution of an inexpensive and readily available oxidizing acid such as nitric acid or chromic acid for a period of time which is found suflicient to completely passivate the surface of article afterwhich it is withdrawn from the solution and cleansed in any suitable fashion.

Preferably the article is plunged into a boiling solution since the maintenance .of the solution at the boiling point assures a constanttemperature and since furthermore, as a result of the Finishes intermediate these two are.

boiling" action minute bubbles of gas attached to the surface of the article are expanded as the article is heated, due tothe immersion thereof in thehot solution, and are then effectively removed by the vigorously moving body of liquid. The removal of surface gases exposes the continuous surface of the article or manufacture and permits a complete passivating treatment of this entire uninterrupted surface.

Under the oxidizing action of the solution a very thin transparent oxide film is formed on the surface of manufacture. This film is hard, continuous and effectively prevents penetration of corrosive agents and thus precludes the inception and growth of corrosion along the surface so passivated.

It may at this point he noted that the effectiveness of the passivating treatment is not gained at the expense of the character of the surface of the article. There is no attack, etching or minute pitting of the surface by the' passivating treatment so that the desired finish given the article immediately prior to treatment is fully preserved.

Since there is no attack on the article by the passivating solution there is then no impoverishment of the solution or depletion of the oxidizing acid. Thus the treatment is effectively'carried out with a minimum of materials which have a maximum life and which are uniformly eflicient and dependable. A

As a further advantage of my passivating treatment there is a complete absence of corrosive vapors and noxious fumes about the tanks or vats in which the dipping part of the process is carried out so that possible danger to workmen and damage to operating equipment is reduced to a-minimum.

As a specific example of my process for treating a corrosion resistant article, a manufacture of the class indicated is dipped into a boiling bath of 0.5'per cent nitric acid for a period of some 30 minutes after which the manufacture is removed and washed in running water then dried in any convenient manner.

While best results in passivating rustless iron of the analysis indicated are achieved when a 0.5 per cent solution of nitric acid is employed, very good results are achieved where the strength of the solution is between 0.1 per cent and 4.0 per cent. Likewise while a bath of boiling solution is preferable good results are attained where the treating bath or solution is maintained at a temperature somewhat beneath the boiling point of the solution.

Where rustless iron and steel articles of lower chromium grades are to be treated highly beneficial results are achieved by employing a passivating solution comprising a weak oxidizing acid such as nitric acid, chromic acid or the like together with a weak solution of a stable oxidizing salt such as sodium dichromate, potassium di- .chrdmate or the like. The presence of the oxidizing salt effectively prevents any minute attack or etching of the metal article. A solution of about.0.5 per cent nitric acid and about. 0.5 per cent potassium dichromate gives best results although good results are achieved .where the range of concentration of both of these ingredients is'extended from about 0.1 per cent to about 4.0 per cent or even more where the analysis of the article and the condition of the surface warrants an increased concentration of acid and salt.

By my passivating treatment a corrosion resistant article is rendered free from the inception and growth of a corrosion product over the surface of the article under the many varying conditions of actual practical use.

For example, automobile trim treated. in accordance with my method successfully withstands a standard salt spray test (4% sea-salt solution) of 100 hours as well as a 100-hour boil in a 0.5% solution of three parts of sulfuric acid, one part of nitric acid, without rusting or corroding.

Thus it will be seen that there has been provided in this invention an art of treating corrosion resistant irons and steels in which the various objects hereinbefore noted, together with many thoroughly practical advantages, are successfully achieved. It will be seen that the process is eflicient, economical and thoroughly reliable and that the product is effectively prevented from the inception and growth of corrosion without in any way affecting the other desirable characteristics of the alloy metal.

While in the above illustrative embodiment of my invention a smooth satin finish was given the product immediately prior to the passivating treatment described, it will be understood that such surface finishing may be omitted, where the steel without etching the surface of the metal,

which includes subjecting said iron and steel to the action of a solution of 0.1 per cent to 4.0 per cent of one of the group consisting of nitric and chromic acids and 0.1 per cent to 4.0 per cent of one of the group consistingof sodium and potassium dichromate..

2. The art of passivating rustless iron and steel without etching the surface of the metal, which includes subjecting said rustless iron and steel to the action of a hot solution of 0.1 percent to 4.0 per cent nitric acid and 0.1 per cent to 4.0 per cent potassium dichromate.

3. The art of passivating rustless iron and steel without etching the surface of the metal, which includes subjecting said rustless iron and steel to the.action of a boiling solution of about 0.5 per cent nitric acid and 0.5 per cent potassium dichromate. I

4. The art of preventing corrosion of rustless iron which includes subjecting said iron to the action of a hot bath of a 0.1 per cent to 4.0 per cent solution of chromic acid together with a 0.1 per cent to 4.0 per cent solution of sodium dichromate.

5. The art of preventing corrosion of rustless iron and steel articles without etching the surfaces of said articles, which comprises cleaning and polishing the surfaces of said articles, and then subjecting said articles to the passivating action of a hot bath of 0.5 per cent nitric acid and. 0.5 per cent potassium dichromate. 

